The present invention relates to a method and an apparatus for aligning the edges of adjacent sheets arranged in a stack, such as paper sheets, paperboards, cardboards and the like.
In many fields of human endeavor, such as in the paper manufacturing, printing and packaging industries, there is a need for accurately aligning the edges of sheets arranged in a stack, for instance, for subsequent printing, trimming or binding operations. In many instances, particularly when the sheets are already provided with fresh printed indicia thereon, it is also necessary to assure that the individual sheets of the stack are separated from one another by an air cushion at least during the alignment operation so as to prevent smudging of the fresh print or transfer thereof from one sheet to another as the individual sheets conduct movement relative to one another.
To achieve these objectives, it is already known to provide an apparatus having a base and a plate on which the individual sheets are being stacked or to which the stack is supplied, and to subject the stack to oscillatory motion by vibrating the plate. However, experience has shown that such an apparatus obtains unsatisfactory results due to the fact that all of the individual sheets are subjected to the same oscillatory motion so that the sheets show indifferent behavior with respect to one another, that is they move in unison, at least up to the time that some of the leading edges of the sheets in the direction of movement of the stack abut against an abutment element. In addition thereto, there is a tendency of the individual sheets to adhere to one another, either due to the friction between the sheets or to the electrostatic charge thereon, or for any other reasons. As a result of this the movement of some of the sheets may terminate before the leading edge thereof reaches the abutment element.
There are also already known apparatuses in which the problem of the individual sheets adhering to one another during or subsequently to the stacking operation has been avoided or at least substantially so by introducing air between the individual sheets so as to in effect form air cushions between the same. However, because of the inherently low stability of the paper sheets, paperboards or cardboards, the alignment of the edges of the sheets while air is introduced between the individual sheets has encountered considerable difficulties. As a result of this, the heretofore known devices for aligning the edges of the adjacent sheets have found only a limited application, particularly because of the cumbersome and unreliable operation thereof.